The main goal in the application technology of motor bearings is to choose the right bearings and apply them appropriately. On one hand, the use of bearings needs to meet performance requirements, while on the other hand, it needs to consider economic performance under the premise of satisfying the performance.
Previous technical articles have focused more on how the selection and application of motor bearings meet their technical performance factors. In fact, the correct technical selection of bearings is also a design of bearing economic performance.
Once the selection of motor bearings is determined, the cost of these bearings is established and the size, precision, and material of the surrounding components of the bearings are also determined after the selection design process is completed. Under such determined design, the cost of bearing application is roughly set.
Optimization of procurement and usage based on this can only be a bonus and cannot make fundamental changes. If the selection is inappropriate, the bearing is not fully utilized, and even if the procurement is cheaper, it will not have a significant effect.
Today’s article begins with the economic performance of bearing selection, discussing how bearing selection can save money.
Avoid choosing bearings too large or too small
In fact, this is a very old topic. However, if we delve into the details, not all engineers understand it.
Firstly, over-sizing in bearing selection. So, what is “large”, and how much is “too”? What are the symptoms of over-sized bearings? How does it impact economic and technical performance? We attempt to explain:
Issue of over-sizing in bearing selection
It is very easy to understand that if the bearing selection is too large, then the capacity of the bearing is not fully utilized. Therefore, this part of the unutilized bearing capacity is a waste. In other words, if a smaller bearing can satisfy the technical performance, then choosing a larger bearing will cause more waste.
This is the hard cost of the bearing, a larger bearing is definitely more expensive than a smaller one (except for custom bearings). Therefore, if considering economics from a design perspective, the first thing is to consider whether the bearing has been chosen “too large”.
So, how “large” is considered “large” in the case of over-sizing bearing selection, and how much is “too”? This is a baseline that engineers must understand.
The size of the shaft and bearing selection is based on a comprehensive consideration of the load the bearing bears in the motor, rotation speed, and other factors. Common methods are the calculation of bearing lifespan and the calculation of minimum bearing load.
In “Motor Bearing Application Technology” and “Motor Bearing Failure Diagnosis and Analysis”, the calculation of the upper limit and lower limit of bearing load capacity in bearing selection is specifically introduced.
Under given working conditions, the upper limit of bearing load capacity (that is, the maximum load capacity of the bearing under the current load, in other words, if a bearing with a load capacity greater than this is chosen, problems will arise) is the minimum load capacity of the bearing.
Under given working conditions, the lower limit of bearing load capacity (that is, the minimum load capacity of the bearing under the current load, in other words, if the chosen bearing load capacity is less than this value, problems will arise) is the bearing load capacity requirement to meet the bearing life requirements.
From this introduction, it is clear that a hard rule for oversizing bearings is the minimum load capacity. If the minimum load required by the selected bearing is larger than the actual load in operation, the bearing is undoubtedly oversized.
In this case, if the bearing operates for a long period, the rolling elements inside may not achieve pure rolling and issues such as slipping and overheating can arise. This not only wastes the load capacity of the bearing and the purchasing cost, but also shortens the bearing life, increases the risk of bearing failure, and thus wastes maintenance costs.
You may ask, is the minimum load the most accurate standard to judge if the bearing size is too large? Is this the most economical standard?
The answer is not necessarily so.
In fact, if the minimum load required for the selected bearing operation exceeds the minimum load in the actual working condition, the bearing is very prone to damage. At this point, the oversized bearing selection already fails to meet the technical requirements.
Since we are discussing “how to save money”, we need to delve further. Besides the minimum load requirement, can we further control the economic performance? Therefore, we continue the previous topic “oversizing problem in bearing selection”.
The “oversizing” problem in bearing selection
As mentioned before, if the load capacity of the bearing is too high and it can’t even meet the minimum load, it’s certainly a waste of bearing capability, and therefore a smaller bearing can be chosen. As we all know, the smaller the standard bearing in regular batch production, the lower the cost. Therefore, choosing a smaller size directly results in cost savings.
So, how small should the bearing be? This is related to how much it has been oversized in the bearing selection.
Our primary principle in selection is to meet technical performance, and the most cost-effective solution that meets technical performance is the best one. When it comes to bearing selection, the smaller the bearing that meets the technical requirements, the better. By the way, the smaller the better, not only saves money economically, but also is better in terms of mechanical structure compactness.
Speaking of the bearing selection being smaller the better, we need to know how small is appropriate.
The “undersizing” problem in bearing selection
Under the premise of meeting technical requirements, the “smallest” bearing selection plan is the most economical. But once the “smallest” limit is breached, the problem of “undersizing” in bearing selection will arise.
The previous article introduced the upper limit of bearing load capacity, which stipulates that technically the bearing cannot be too large. Then, the lower limit of the bearing load capacity accordingly stipulates the limit of the bearing not being too small.
The commonly used method is the “bearing life verification calculation”. This calculation is often misunderstood by engineers as a “fortune-telling” method, which is actually incomplete. The bearing life verification calculation is actually a verification of the bearing load capacity. It verifies whether the load capacity of the bearing can meet the requirements of the actual load. In other words, it is the minimum load capacity requirement of the bearing under given load conditions, or the “small” limit of the bearing.
If the bearing load capacity is less than this requirement, then under the given working conditions, the bearing will not be able to reach the calculated life, so this is a hard technical requirement.
“Economic” Selection of Bearing Size
Considering the above-mentioned maximum and minimum limits for bearing selection, engineering technicians should understand the constraints within a given operating load. The bearing size should not exceed a certain limit, and cannot be smaller than another.
Within these boundaries of bearing selection, the closer to the lower limit, the smaller the bearing size chosen, and the lower the “unit price” of the bearing. From the perspective of selection size, this kind of bearing selection is more economical.
In practical engineering, engineers need to estimate factors such as changes in operating conditions, improper application, and many other factors. Therefore, a certain margin is reserved based on the life expectancy of the bearing. This is the most economical selection scheme for bearing selection.
The above only discusses the choice of bearing size. In fact, in the issue of bearing selection, there are also choices in form, design of bearing configuration, and many other factors, all of which directly affect the economic performance of bearing selection.
More will be gradually unfolded in the future.